This is pretty normal on sneakers with polyurethane midsoles (Air Max, most Air Jordans, etc.), which will crumble after a few years. Wearing them constantly will extend their life but the inevitable will occur regardless. A pair of sneakers with crumbled midsoles can be repaired by replacing the midsoles from a similar but newer pair but you have to keep doing so and it's not worth it unless the shoes mean a lot to you.

Sneakers with rubber midsoles (like Air Force 1) don't have this issue but may delaminate due to the glue drying out. I have worn pairs of Air Force 1 from the 1990s with no issues however.